Unlike cattle, most sheep are seasonal breeders, and are not sexually active outside of their normal breeding season. When that breeding season is depends on the breed, the locale and the climate.
Some breeds of sheep remain sexually active all year round. Dorset is one, I think (someone will correct me and/or tell us if it's polled or horned or both) - some producers use this to be able to produce lamb all year round, more commonly to have two lambings, one spring and one autumn. (Not the same ewes, I hasten to add! Two flocks, one spring lambing one autumn.)
BH used to leave a tup in with a few ewes over the summer, just as part of his general nature and weather sensing input. The earliest he ever got a lamb here was Jan 4th. We don't do that now as we are using Charollais tups and if not year-round breeders they certainly start earlier than our Texels!
HTH
Sally